Fluorescent Nanoparticles Stabilized by Poly(ethylene glycol) Containing Shell for pH-Triggered Tunable Aggregation in Aqueous Environment

Abstract

Fluorescent silica nanoparticles decorated with a responsive shell, a mixed polymer brush, were synthesized. Specifically, a poly(2-vinylpyridine), P2VP, and poly(ethylene glycol), PEG, binary polymer brush was synthesized on silica nanoparticles via the "grafting to" technique. The selection of the components (PEG and P2VP) for the responsive brush shell was motivated by potential biomedical applications. Poly(glycidyl methacrylate), PGMA, labeled with Rhodamine B, RhB, was used to form a reactive and fluorescent shell on the nanoparticle surface. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change in pH. At lower pH, aqueous dispersions of the particles are stable, since PEG and P2VP are water-soluble, extended and contribute to the steric and electrostatic mechanisms of colloidal stability. An increase of pH to 6 causes a slow aggregation as a consequence of the hydrophobic attraction between the collapsed and almost nonprotonated P2VP macromolecules. The aggregation was well controlled and occurred within 90-120 min of the pH change. The aggregation was fully reversible after the decrease in pH. The pH variation did not quench the fluorescence of the colloidal suspensions. The pH-tunable aggregation of the fluorescent nanoparticles could find diverse applications for labeling and contrasting of cells and tissues when the size of the label and the intensity of the optical signals can be tuned by and related to the pH of the host environment.

Polymer Brushes by the “Grafting to” Method

“…[14,15] Other options for minimizing the excluded volume interactions, and thereby producing thicker (10-30 nm) grafted layers, are to conduct the grafting from a ''theta'' solvent [16] or from a concentrated polymer solution. [17,18] Synthesis of brushes on flat substrates, [12,[19][20][21] porous structures, [22,23] fibers/textiles, [24][25][26][27][28][29][30][31] and nanoparticles [28,32,33] using the ''grafting to'' procedures has been reported. In addition to synthesis of the uniform grafted layers the ''grafting to'' approach can be readily employed to synthesize patterned anchored polymer layers and brushes with variations in grafting density along a substrate ( Figure 1).…”

“…[12,15,[107][108][109][113][114][115][116] The layers were synthesized on flat substrates and surfaces of nanoparticles. [12,28,32,116] For example, Iyer et al [12] reported PS grafting to a silicon surface modified with a thin layer of PGMA. The utilization of macromolecular layer containing epoxy groups yielded dense and homogeneous PS layers grafted onto a silicon wafer.…”

“…Various combinations of hydrophobic/hydrophilic/anionic/cationic mixed brushes have been synthesized via sequential ''grafting to'' methods using PGMA as the macromolecular anchoring layer. [10,32,34,[121][122][123][124] The anchoring layer was used to synthesize a multicomponent mixed polymer brush that is capable of reversibly exposing nanometer-sized hydrophobic fragments at its hydrophilic surface and of tuning, turning on, and turning off the hydrophobic interactions. [125] The brush was composed of a hydrophilic non-ionic polymer (PEG) and an amphiphilic block copolymer (PAA-b-PS).…”

Surface Plasmon Modes of Sandwich-Like Metal–Dielectric Nanostructures

“…Carboxy-terminated P2VP was used to obtain the surface anchored layer by the "grafting to" method. The carboxyl end groups in the P2VP chains have been shown to be capable of reacting with epoxy groups present in the PGMA film [49,50,52]. The thickness of the grafted layer was on the order of 10-15 nm.…”

Facile fabrication of amphiphilic gold nanoparticles with V-shaped brushes from block copolymers with a trithiocarbonate group as the junction

“…In order to stabilize the AuNPs, smaller organic molecules or polymers are usually grafted on the surface of AuNPs. Recently environmentally responsive nanoparticles grafted with mixed brushes have attracted many researchers [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The mixed brushes on the surface of the nanoparticles can undergo conformation reorganization upon external stimuli such as pH, temperature and solvent, leading to microphase separation, change of solubility in solution and sometimes phase transfer between different solvents [19][20][21][22].…”